For pushing the Scom agent to a ISA server you need to do some things.
First we have to make a rule and a protocol rule and then you can push the agent. here is a how to do this with a "nijntje manual "
Open ISA server
Create Protocol Rule
Use Port 5723 TCP ! this is the default Scom Agent port
Go to the scom server do a discovery pick the ISA server and push the agent
As you can see it works BUT but I tricked you. if you do this it may not work.
In the scom account settings there is a setting this account is a remote local account USE this and the agent push will succeed